Group of nations supporting aid fixed for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'group of nations supporting aid fixed' is the wordplay.
'group of nations' becomes 'eu' (European Union).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'fixed' is an anagram indicator.
'aid' with letters rearranged gives 'adi'.
'eu' after 'adi' is 'ADIEU'.
'for' is the link.
(Other definitions for adieu that I've seen before include "farewell message" , "Leave" , "Goodbye to the French" , "All the best" , "A valediction" .)